Field of the Invention
The invention relates to a fixing device mounted on an image forming device such as an electrophotographic copying device or a printer, an elastic roller included in this fixing device, and the fixing device including the elastic roller.
Description of the Related Art
A fixing device included in an electrophotographic toner-image forming device includes a heating member, which is heated by a heater and heats an image by coming into contact with an image carrying surface of a recording medium, and a pressing member disposed opposite to the heating member and forming a fixing nip (hereinafter referred to as a “nip portion”) in cooperation with the heating member.
In recent years, techniques for enhancing heating efficiency of a fixing device and reducing the start-up time (warm-up time) of a fixing device have been developed for energy saving. One example of such techniques is a use of porous silicone rubber having some voids as an elastic layer of a pressing member. Such a pressing member impedes thermal conduction. Thus, the pressing member prevents the heat from the heating member from transmitting through itself to other components, whereby the recording medium on which the toner is held can be efficiently heated at the nip portion.
The pressing member that impedes thermal conduction, however, has a disadvantage in that while a recording medium is passed through the nip portion, the pressing member accelerates temperature rise in an area of the pressing member with which a recording medium does not come into contact (hereinafter this area is referred to as a “non-sheet-passing area”), more specifically, in widthwise both end portions of the pressing member, that is, it accelerates “temperature rise in a non-sheet-passing area”.
The temperature rise in a non-sheet-passing area is such a phenomenon that the temperature rises in an area of the nip portion with which a recording medium does not come into contact since the heat from the heating member is not transferred to the recording medium or toner on the recording medium. The temperature rise particularly occurs when recording media are successively passed through the nip portion. The temperature rise in a non-sheet-passing area may degrade the quality or distort the shape of the heating member or the pressing member of the fixing device. Moreover, when the nip portion that is in an excessively heated state after small-size recording media are passed therethrough receives a larger-sized recording medium, excessively molten toner may cause offset.
To address this situation, Japanese Patent Laid-Open No. 2012-37874 discloses a method for reducing the temperature rise in a non-sheet-passing area by enhancing the start-up performance with dispersion of voids in an elastic layer of the pressing roller and by enhancing the thermal transmission of the pressing roller in a direction along the rotation axis of the pressing roller (the direction is hereinafter referred to as an “axial direction”) with orientation of a needle-shaped filler in the axial direction.